Recent research strongly suggests that the ionosphere plays a crucial role in the dynamics of space weather. Although the ionosphere is by volume only a small fraction of the magnetosphere, it serves as a variably conducting boundary, modulating the global electrodynamic circuit in crucial ways. A striking example is the behavior of intense aurora, which have recently been discovered to occur only when the background ionospheric conductivity is low. It is now clear that auroral acceleration occurs at the interface between the ionosphere and the magnetosphere and is controlled by magnetospheric-ionospheric coupling, with the solar cycle variations arising from a surprising source: variations in solar EUV flux. The discovery of diverging electric fields with their possibly corresponding black aurora provides a new symmetry to magnetosphere-ionosphere coupling processes. The far-reaching scope of the ionosphere in space weather problems is illustrated here in several ways. Ionospheric convection is suggested to be a major player in space weather, by creating global coherence in the magnetosphere on timescales not otherwise practical. Even a problem seemingly as far removed as possible from the ionosphere, namely, that of charge neutrality in polar rain (superthermal solar wind electron) entry into the distant magnetotail, is shown to be coupled to the problem of polar wind outflow from the ionosphere. ¿ 2001 American Geophysical Union